In 2009, the smartest invention of recent years was launched: worth 11 distinct patents with extremely high scientific and economic value. Take a look:
Dyson Air Multiplier is a fan without visible blades. The biggest values of this gadget lie on being safe to curious pets and children, easy to clean and obviously technologically revolutionary.
Here you can see Dyson Air Multiplier next to his newest fan, back in 2010. Besides being undoubtely something I loved to play with, the first thing that came to my mind was the possible immediate application in closed circuit wind tunnels.
Why not? One of the biggest problems in aerodynamic tests is the control of the turbulence intensity in the test section of the wind tunnels. Turbulence — fluid velocity fluctuations, on small or large scales — may be due to the different structural components that conduct the flow, from the walls themselves to directional deflectors. However, the biggest turbulence generator in closed circuit wind tunnels are the fan blades.
Ok… enough of endeavors… how does it work?
Not that complex, which makes it even more interesting.
As we can see in the video, the device drives air through an axial rotor located at its base. The air flows into the ring and exits in the form of a jet of air through a narrow slot along the entire inner surface of the ring. Because the air has viscosity, the jet drags the ambient air (which was still). This phenomenon is called viscous entrainment. As this ambient air was drawn towards the jet, more ambient air should take its place, promoting the flow of ambient air (which did not pass through the rotor at the base) towards the ring axis and towards the jet.
Not only that: the curved surface shape in the jet region causes the airflow to be curved as well, thus reducing pressure not only because of ambient air acceleration, but also because the flow is curved (pressure always reduces towards the center of the curvature radius, as a fluid dynamics principle). This causes even more ambient air to be accelerated towards the ring axis and towards the jet, multiplying all these effects and bringing in more ambient air…
Dyson claims that the fan pushes 15 times the amount of air that is sucked into the base and exits through the slot. Do not confuse this, however, with an efficiency of 1500%: the ambient air only accelerates at the expense of a deceleration of the air that leaves the slot (the kinetic energy of the air that leaves the slot is transferred to the ambient air, through viscosity). This is not a more efficient device, but a safer and, certainly, more beautiful fan!
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